Burkitt lymphoma risk shows geographic and temporal associations with Plasmodium falciparum infections in Uganda, Tanzania, and Kenya

Significance Endemic Burkitt’s lymphoma (eBL) is the most common pediatric cancer in malaria-endemic regions of sub-Saharan Africa and is universally fatal if untreated. Previous research has suggested an etiological link between malaria exposure and eBL. In this study, we used spatially detailed data on malaria parasite infection and eBL incidence in sub-Saharan Africa to assess the strength of this association. We found each additional 100 lifetime P. falciparum infections were associated with a 39% increase in age-specific risk of eBL, suggesting that malaria reduction may substantially reduce eBL incidence and mortality. Because the impact of malaria on eBL appears to be cumulative, evaluation of these efforts should account for lags between declines in malaria incidence and eBL incidence.


Supplementary text Figures S1 to S9 Table S1 Methods
Evaluating temporal trends in estimated P. falciparum incidence and eBL incidence To evaluate temporal trends in estimated P. falciparum incidence, we used a gaussian regression model with a fixed effect for year and country and a random effect for district.
Year was included as a factor. Although estimated P. falciparum incidence was much lower in 2016 than in 2000, it has not been decreasing monotonically and increased between 2015-2016. While Tanzania has seen a constant slow decline in estimated P. falciparum incidence, Uganda saw an initial decrease followed by a spike in 2009. In 2016, both Uganda and Kenya saw an increase in estimated P. falciparum incidence from 2015, although estimated P. falciparum incidence is still lower than in 2000.
To determine if eBL incidence decreased in the study regions over the study period, regardless of P. falciparum, we employed a negative binomial regression with fixed effect for year, country, age, and sex. A random effect was included for the district, and the population was included in an offset term. We found age, included as a factor, to have a statistically significant relationship with eBL. Using age 10 years as the reference group, children 3 years and under or over 14 years had a lower risk of eBL. Children age 11 years had the highest risk and children 4-14 years had no statistically significant risk difference from children age 10. Risk did not differ significantly by year over the study period, although Tanzania had a much lower risk ratio (RR: 0.25, 95% CI: 0.11 -0.54) than Kenya, while Uganda and Kenya did not significantly differ. Males had a 71% higher risk ratio than females (95% CI: 1.45 -2.03).

Modeling the Association between eBL and Annual P. falciparum
Additional models examined the effect of individual years of estimated P. falciparum incidence on eBL incidence. First, 11 negative binomial regressions were performed with the same covariates as the main model from the text. Instead of including the cumulative estimated number of infections, the average number of infections n years ago was included in 11 separate models ( Figure A9). Next, the estimated number of P.
falciparum infections from the past 10 years and the current year were included in the same model ( Figure A10). The 11 individual models showed a statistically significant protective effect of P. falciparum incidence 1 and 2 years prior, while P. falciparum incidence 9-and 10-years prior was associated with increased eBL risk. However, a district's estimated P. falciparum incidence is strongly correlated from year to year, making it difficult to interpret these results. The model with all years included showed no statistically significant effects but is also likely skewed by the correlation of covariates.
These models demonstrate that, on their own, individual annual estimates of P.
falciparum burden are poor predictors of eBL incidence and indicate the need for a metric of malaria burden over the life course.

Assessing the sensitivity of the relationship between eBL and Annual P. falciparum
To determine the sensitivity of the relationship between eBL and the annual P. falciparum incidence given the uncertainty in the 2−10 estimates, we recreated the main analysis using both the lower and upper bounds of the 95% confidence intervals of the 2−10 estimate. Using the lower estimate of annual P. falciparum incidence, we found that for every 100 cumulative P. falciparum infections, the risk of eBL increases 215% (95% CI: 1.80 -5.39). Using the higher estimates, 100 cumulative P. falciparum infections is associated with a 3% increase in the risk of eBL (95% CI: 0.99 -1.06).
Although the highest estimates of 2−10 do not result in a statistically significant association between eBL and cumulative P. falciparum incidence, point estimates demonstrate a positive association and indicate that only extreme deviation from the mean estimates of PfPR2-10 nullify the association.       , 5, 8, 11, and 15 were selected to demonstrate the effect of age on risk. When compared to a 10 yo with 100 cumulative infections, the simulation shows that the IRR for eBL attains statistical significance (i.e., estimated IRR 95% lower confidence interval is above 1) among 5-11 yos, whereas by age 15, there is no statistically significant difference from the reference group regardless of P. falciparum exposure.

Fig. S9
. Incidence Rate Ratios of lagged P. falciparum infection incidence at 0 through 10-year lags, with each lag included individually. The IRR associated with prior years' estimated P. falciparum incidence from 10 years ago until the present year when each variable is individually included its own model demonstrates a statistically significant protective effect of P. falciparum incidence from current year and 1 year prior. These findings demonstrate that a single year's measure of P. falciparum infection is inadequate to represent the lifetime P. falciparum burden, because children under two years old, who only have P. falciparum exposure in the current and previous year, are unlikely to develop eBL regardless of exposure burden, introducing a cohort effect that may confound results. Additionally, the immune response to P. falciparum alters the probability of successful infection in later years, leading to potential misclassification of the exposure and biased results. The P. falciparum incidences 9-and 10-years prior are associated with an increased IRR; however, these results should be interpreted with caution because many of the age groups with the highest eBL incidence (5-11 yo's) lack a 9-and 10-year lagged exposure.